Abdullah bin Umar (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "The night prayer is two rak'ahs at a time. When you sense that the morning is about to overtake you, then pray witr with one rak'ah." A man asked Ibn Umar (may Allah be pleased with them both): What is meant by "two"? He said: Saying the salam after every two rak'ahs.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4571